This invention relates to a high pressure flow control valve. More particularly, the invention relates to an improved sealing system for pressure equalization of the valve chamber.
Ball valves include a central bore for controlling passage of fluid through a flow line with a ball being disposed within a valve chamber and rotationally supported by a pair of spaced annular seat carriers. Each of the seat carriers encircles a valve seat with the seats sealing the upstream flow passage from the downstream flow passage. A common problem is that the upstream seat tends to become extruded from the seat carrier or blown out of the valve chamber if the valve is opened when a large differential pressure exists, i.e., high upstream pressure and low downstream pressure. To solve this problem, the prior art has suggested means for pressure equalization between the upstream flow passage and the valve chamber. For example, it is known to provide slots on the outside diameter of the seats. When pressure differential causes the upstream seat to move against the ball with the ball moving downstream a short distance, pressure in the valve chamber is equalized through the slots. As soon as the pressure differential in the valve chamber is eliminated, the upstream seat returns to its original position.
U.S. Pat. No. 3,778,029; incorporated herein by reference, discloses a two-way ball valve including a sealing system that seals on the downstream seat while equalizing pressure on the upstream seat. The valve includes a pair of spaced seats disposed on each side of a flow control ball. Each seat is encircled by a seat carrier with the upstream seat carrier intentionally leaking in the downstream direction and the downstream seat providing a seal between the valve chamber and the downstream flow passage. That is, only the downstream seat actively seals the flow line. When it is desired to equalize upstream pressure into the valve chamber, a U-shaped seal is provided on both seat carriers. This U-shaped seal is positioned within an annular recess in an outwardly facing corner of the seat carriers adjacent the inner wall of the valve chamber. This sealing system seals the downstream side of the flow control ball, equalizes pressure between the valve chamber and the upstream seat and reduces the turning torque required to open the valve. A disadvantage of this pressure equalization system, however, is an excessive cost for the U-shaped seal ring, which must be specially constructed for this application.
Another disadvantage of prior art ball valves is they generally can be used only in low pressure situations, i.e., less than 3000 psi, because the elastomeric valve seats deform excessively under high compressive loads with high differential pressures. Ball valves for use in high pressure situations, i.e., 3000-10000 psi, generally use trunnion mounted control balls. A ball valve having a trunnion mounted flow control ball has reduced operating torque but is more costly to manufacture and does not seal as well as a valve having a floating control ball.
Accordingly, there remains a need for a flow control valve that can be used in high pressure applications having an inexpensive sealing system that seals on the downstream seat only and equalizes upstream pressure into the valve chamber.